Moisture sensors for dew-point measurement are humidity-sensing capacitors. Currently most moisture sensors on the market use porous amorphous- or γ-Al2O3 films, formed by anodization in sulfuric acid solution. For simplicity, the amorphous/γ-Al2O3 films based moisture sensors are called γ-Al2O3 sensors. γ-Al2O3 sensors exhibit long-term drift of calibration curves. The mechanism for long-term drift of γ-Al2O3 sensors is caused by the phase change of γ-Al2O3, i.e. amorphous- or γ-Al2O3 changes to γ-Al2O3.H2O (boehmite) when it is exposed to moisture. This irreversible phase change causes volume expansion of aluminum oxide, resulting in gradual decrease in pore diameter and porosity so that its moisture adsorption capability is reduced. These γ-Al2O3 moisture sensors have to be calibrated constantly and also to be stored in a very dry environment. The accuracy and reliability of γ-Al2O3 moisture sensors are poor and it is also very inconvenient for users to use them. It is therefore very important to develop or invent a solid-state moisture sensor that is long-term stable and can be stored anywhere without special care.
In order to fabricate moisture sensors with high precision and long-term stability, γ-Al2O3 must be replaced or covered by a highly stable and moisture-sensing dielectric material. Toward this end, novel moisture sensor disclosed in this document use SiO2 as a moisture sensing material. Actually SiO2 is a unique material, which has been used in silicon-based integrated circuits for several decades. It is also sensitive to moisture because SiO2 is hydrophilic. Therefore, SiO2 can be used for moisture sensors if high-quality porous SiO2 films can be obtained. However, it is very difficult to obtain high-quality porous SiO2 films. The highest quality SiO2 is formed by thermal oxidation of silicon. However, thermal SiO2 can be grown only on silicon substrates, which cannot be used on other substrates. SiO2 films can be deposited on other substrates by conventional deposition techniques, e.g., chemical vapor deposition (CVD), sputtering, and electron beam evaporation. However, it is well known that these SiO2 films are non-stoicheometric with poor quality, low breakdown fields, and many defects. Furthermore, using the above methods one cannot deposit SiO2 films conformally in high-aspect-ratio pores and therefore are not useful for moisture sensor fabrication. In recent years, a new deposition method, atomic layer deposition (ALD) emerged. ALD deposition of SiO2 films can circumvent these problems because the material is deposited one atomic layer at a time, which can deposit thin SiO2 films conformally on the inner surfaces of pores, and also controls the thin film thickness precisely at atomic scale.